Interface-limited injection in amorphous organic semiconductors

We examine electron transport in the archetype amorphous organic material tris(8-hydroxyquinoline) aluminum (Alq₃). It is established that for Al, LiF/Al, and Mg:Ag cathodes, injection processes at the metal/organic contact dominate the current-voltage characteristics. We find that transport is also injection-limited at low temperatures, but that the cathode dependence of current-voltage characteristics at T=30 K is substantially reduced, raising doubts over metal-to-organic injection models that depend on the cathode work function. Given that ultraviolet photoelectron spectroscopy measurements show a shift in the vacuum potential at the metal/Alq₃ interface of ∼1 eV, we investigate the impact of interfacial dipoles on adjacent molecules in the organic film. Consequently, we propose that injection is limited by charge hopping out of interfacial molecular sites whose energy distribution is broadened by local disorder in the interfacial dipole field. We derive a general analytic model of injection from interfacial states and find that it accurately predicts the current-voltage characteristics of transport in Alq₃ over many orders of magnitude in current and over a wide range of temperatures. The model is extended to other amorphous organic semiconductors and is found to be applicable to both polymers and small molecular weight organic compounds.